RA-226 IN THE BLACK-SEA AND SEA OF MARMARA

Water column distributions of Ra-226 were determined at stations in the Sea of Marmara and the Black Sea as part of the 1988 Joint U.S.-Turkish Black Sea Expedition. Black Sea surface water Ra-226 concentrations were a factor of three to four lower than measurements made 20 years earlier. The most likely cause is increased removal of Ra-226 and Ba [35] due to increased surface biological activity; a secondary effect is decreased fluvial discharge and related dimunition of inputs by desorption from fluvial suspended sediments. The amount of Ra-226 missing from the surface waters of the Black Sea over this period is accounted for in the high-porosity surficial "fluff" sediment layer. Throughout the Black Sea, depth profiles of Ra-226 exhibited pronounced maxima of approximately 25 dpm/100 L at about sigma(theta) = 16.2-16.3, in the vicinity of a bacterial maximum, but slightly shallower than the total dissolved Mn and Fe maxima (sigma(theta) = 16.4-16.5) reported by Lewis and Landing [38]. While the Ra-226 maximum may, in part, be linked to the cycling of Mn and Fe oxyhydroxides near the O2/H2S interface, its distribution appears to be more plausibly explained as a result of the microbial breakdown of particulate organic matter and the subsequent release and partial dissolution of associated barite in this region. A simple steady-state two-box model has been used to obtain a semiquantitative understanding of the behavior of Ra-226 in the Black Sea. By incorporating reasonable estimates for the input and removal of Ra-226 in the Black Sea, an excellent agreement between predicted and observed (1988) Ra-226 concentrations was achieved. The model suggests that the dominant variables controlling the distribution of Ra-226 in the Black Sea are riverine input and cycling with Ba.